Enteric bacterial diseases caused by Shigella remain a serious and frequent cause of morbidity and mortality, including decreased growth and mortality, throughout the world, especially in infants and children of developing countries. In addition, Shigellosis occurs in healthy individuals in developed countries under conditions of crowding, such as recruits in the armed forces or institutions where people are treated for chronic diseases. Despite a century of study after discovery of Shigellae, there are no licensed vaccines for prevention of diseases caused by these pathogens. The major cause of this failure to have effective vaccines is that the human protective immune mechanism is not known. We have hypothesized that serum IgG antibodies to the O-specific polysaccharides of Shigella confer protective immunity to Shigellosis. Conjugate vaccines for induction of serum IgG antibodies to the O- specific polysaccharide have been developed and phase I studies have shown that they are immunogenic and elicit similar or higher levels of antibodies to the O-specific polysaccharide as convalescent patients. In addition, the repeating unit and its saccharide components have been synthesized and the immunodominant component of the O-specific polysaccharide identified for Shigella dysenteriae type I.